It is sought more particularly here below in this document to describe problems existing in the field of geophysical data acquisition for analysing the sea-bed (e.g. for oil exploration industry using seismic prospection method). The invention of course is not limited to this particular field of application but is of interest for any technique for performing quality control of measurements of marine mammal vocalization that has to cope with closely related or similar issues and problems.
Regulation agencies encourage or impose the use of a PAM system during seismic survey campaigns. These regulating agencies propose guidelines defining rules to apply during seismic campaigns in order to protect the marine mammals. In particular the guidelines recommend the PAM system to detect the presence of marine mammals in the vicinity of seismic sources, which can be considered as injurious to the marine mammal life (e.g. acoustic disturbances).
In the prior art, a PAM system typically comprises a network of acoustic sensors, such as hydrophones or geophones or accelerometers or vector sensors, arranged along one or several linear acoustic antennas (hereafter referred to “streamers”) towed by a vessel, and adapted for detecting and measuring vocalizations, i.e. acoustic signals emitted by marine mammals. When a marine mammal vocalizes in the vicinity of the network of seismic sensors, these sensors make measurements of the acoustic signal or signals emitted by the marine mammal. A localization computation is then performed from the measurement data collected by the sensors to determine the localization of the marine mammal from the seismic sources.
There are two known families of PAM systems used by the oil prospecting companies:                the integrated PAM systems, which rely on the use of a network of seismic sensors distributed on the streamers towed by the seismic vessel, which seismic sensors being originally adapted to perform geophysics data acquisition, and thus diverted of its function first to make passive acoustic monitoring;        the independent PAM systems, which rely on the use of a network of dedicated acoustic sensors arranged along a linear acoustic line, which is deployed and towed in addition to the existing streamers behind the seismic vessel and entirely dedicated to the passive acoustic monitoring.        
Nowadays, regulating agencies and stakeholders in marine survey operations establish codes of conduct which suggest demonstrating the capabilities of a PAM system to detect, classify and localize the mammal species likely to be encountered in the survey area during the planning phase of a seismic survey.
The lack of knowledge on the performances of the different devices of a PAM system can reduce the accuracy or usefulness of the collected acoustic data by the sensors. Since seismic surveys can be quite time-consuming, there is a need to evaluating, as the survey progresses, the quality of operation of the PAM system. This is referred hereafter to as “quality control” or “QC”.
Moreover, the guidelines also suggest testing the quality of operation of the towed streamers before each deployment to ensure that no malfunction exists. This simply consists in verifying that the sensors of the towed streamers properly work and are well connected thanks to a so called “tap test”, which is done with manually tapping the sensors and verifying they detect such tapping.
In the prior art, a known method for controlling quality of operation of a PAM system during fieldwork consist in measuring background noise during the seismic survey. At low frequency, the background noise is preponderant on the electronics of components, so measuring the background noise allows checking the integrity of the whole data acquisition chain in the towed streamers, and detecting a possible failure of a sensor.
However, this known method is carried out irregularly and requires on the assumption that the background noise remains relatively constant during the seismic survey. In addition, opportunity acoustic sources, the vessel noise and the noise caused by the seismic source firing are used to build performance indicators of the acoustic reception chain. However, as none of these sources are accurately characterized (in particular emission level versus frequency), this method only permits to check that the acoustic measurements are globally representatives. This does not therefore provide satisfactory quality information. Lastly, if a sensor were in default from the start of the survey, this one would not be identified by that background noise test.
In summary, this known method only provides a rough idea of the level of quality of operation of the PAM system.
Another known method of control of quality of a PAM system is based on the use of a source emitting a test acoustic signal simulating a vocalization of a marine mammal. This source is immerged at a known location and depth. This method of control of quality of a PAM system can be summarized as follow:                a first operator on the source side triggers the source to emit a test acoustic signal;        the PAM system obtains acoustic signal measurements;        a second operator on the PAM system side carries out the following steps:                    estimates parameters, characteristic of and/or associated with the test acoustic signal, as a function of the obtained acoustic signal measurements;            compares the estimated parameter with reference parameters characteristic of and/or associated with the test acoustic signal;            obtains at least one piece of quality information, as a function of results of the comparing step.                        
The reference parameters belongs to the group comprising:                characteristics of the test acoustic signal (such as the emission level, the signal shape, the bandwidth, the centre frequency, etc.);        an emission time of the test acoustic signal; and        an emission location of the at least one source.        
However, this known method is fastidious, as it requires the presence of an operator on simulator side and an operator on PAM system side, the operators carrying out the different steps of the process manually. This involves the presence of experienced operators, especially on the PAM system side where the calculations of quality control are carried out by hand. As data are collected in different locations and different formats, the validation process is further heavy and takes time.
In addition, owing to the frequency bandwidth used by the seismic sources, such known method is highly sensitive to phenomena of masking of sensors. Indeed, frequency bandwidth of vocalizations emitted by marine mammals can be, at least partially, common with the frequency bandwidth used by the seismic sources. Then, the detection range for a given marine mammal is highly reduced if its vocalizations illuminate the sensors during a seismic shooting session. This masking phenomenon being not taken into consideration in the known method, quality control results are highly variable and therefore not always reliable.